Network Working Group M. Chen
Internet-Draft W. Cao
Intended status: Standards Track Huawei Technologies Co., Ltd
Expires: February 28, 2013 S. Ning
Tata Communications
F. Jounay
Orange CH
S. Delord
Alcatel-Lucent
August 27, 2012
Return Path Specified LSP Pingdraft-ietf-mpls-return-path-specified-lsp-ping-08.txt
Abstract
This document defines extensions to the failure-detection protocol
for Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
known as "LSP Ping" that allow selection of the LSP to use for the
echo reply return path. Enforcing a specific return path can be used
to verify bidirectional connectivity and also increase LSP ping
robustness. It may also be used by Bidirectional Forwarding
Detection (BFD) for MPLS bootstrap signaling thereby making BFD for
MPLS more robust.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 28, 2013.
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Internet-Draft Return Path Specified LSP Ping August 20121. Introduction
This document defines extensions to the failure-detection protocol
for Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
known as "LSP Ping" [RFC4379] that can be used to specify the return
paths for the echo reply message, increasing the robustness of LSP
Ping, reducing the opportunity for error, and improving the
reliability of the echo reply message. A new reply mode, which is
referred to as "Reply via Specified Path", is added and a new Type-
Length-Value (TLV), which is referred to as Reply Path (RP) TLV, is
defined in this memo.
With the extensions described in this document, a bidirectional LSP
and a pair of unidirectional LSPs (one for each direction) could both
be tested with a single operational action, hence providing better
control plane scalability. The defined extensions can also be
utilized for creating a single Bidirectional Forwarding Detection
(BFD)[RFC5880], [RFC5884]session for a bidirectional LSP or for a
pair of unidirectional LSPs (one for each direction).
In this document, term bidirectional LSP includes the co-routed
bidirectional LSP defined in [RFC3945] and the associated
bidirectional LSP that is constructed from a pair of unidirectional
LSPs (one for each direction), and which are associated with one
another at the LSP's ingress/egress points [RFC5654]. The mechanisms
defined in this document can apply to both IP/MPLS and MPLS Transport
Profile (MPLS-TP) scenarios.
2. Problem Statements and Solution Overview
MPLS LSP Ping is defined in [RFC4379]. It can be used to detect data
path failures in all MPLS LSPs, and was originally designed for
unidirectional LSPs.
LSP are increasingly being deployed to provide bidirectional
services. The co-routed bidirectional LSP is defined in [RFC3471]
and [RFC3473], and the associated bidirectional LSP is defined in
[RFC5654]. With the deployment of such services, operators have a
desire to test both directions of a bidirectional LSP in a single
operation.
Additionally, when testing a single direction of an LSP (either a
unidirectional LSP, or a single direction of a bidirectional LSP)
using LSP Ping, the validity of the result may be affected by the
success of delivering the echo reply message. Failure to exchange
these messages between the egress Label Switching Router (LSR) and
the ingress LSR can lead to false negatives where the LSP under test
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is reported as "down" even though it is functioning correctly.
2.1. Limitations of Existing Mechanisms for Bidirectional LSPs
With the existing LSP Ping mechanisms as defined in [RFC4379],
operators have to enable LSP detection on each of the two ends of a
bidirectional LSP independently. This not only doubles the workload
for the operators, but may also bring additional difficulties when
checking the backward direction of the LSP under the following
conditions:
1. The LSR that the operator logged on to perform the checking
operations might not have out-of-band connectivity to the LSR at
the far end of the LSP. That can mean it is not possible to
check the return direction of a bidirectional LSP in a single
operation - the operator must log on to the LSR at the other end
of the LSP to test the return direction.
2. The LSP being tested might be an inter-domain/inter-AS LSP where
the operator of one domain/AS may have no right to log on to the
LSR at the other end of the LSP since this LSR resides in another
domain/AS. That can make it completely impossible for the
operator to check the return direction of a bidirectional LSP.
Associated bidirectional LSPs have the same issues as those listed
for co-routed bidirectional LSPs.
This document defines a mechanism to allow the operator to request
that both directions of a bidirectional LSP be tested by a single LSP
Ping message exchange.
2.2. Limitations of Existing Mechanisms for Handling Unreliable Return Paths
[RFC4379] defines 4 reply modes:
1. Do not reply
2. Reply via an IPv4/IPv6 UDP packet
3. Reply via an IPv4/IPv6 UDP packet with Router Alert
4. Reply via application level control channel.
Obviously, the issue of the reliability of the return path for an
echo reply message does not apply in the first of these cases.
[RFC4379] states that the third mode may be used when the IP return
path is deemed unreliable. This mode of operation requires that all
intermediate nodes must support the Router Alert option and must
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understand and know how to forward MPLS echo replies.
This is a rigorous requirement in deployed IP/MPLS networks
especially since the return path may be through legacy IP-only
routers. Furthermore, for inter-domain LSPs, the use of the Router
Alert option may encounter significant issues at domain boundaries
where the option is usually stripped from all packets. Thus, the use
of this mode may itself introduce issues that lead to the echo reply
messages not being delivered.
And in any case, the use modes 2 or 3 cannot guarantee the delivery
of echo responses through an IP network that is fundamentally
unreliable. The failure to deliver echo response messages can lead
to false negatives making it appear that the LSP has failed.
Allowing the ingress LSR to control the path used for echo reply
messages, and in particular forcing those messages to use an LSP
rather than being sent through the IP network, enables an operator to
apply an extra level of deterministic process to the LSP Ping test.
This document defines extensions to LSP Ping that can be used to
specify the return paths of the echo reply message in an LSP echo
request message.
3. Extensions
LSP Ping defined in [RFC4379] is carried out by sending an echo
request message. It carries the Forwarding Equivalence Class (FEC)
information of the LSP being tested which indicates which MPLS path
is being verified, along the same data path as other normal data
packets belonging to the FEC.
LSP Ping [RFC4379] defines four reply modes that are used to direct
the egress LSR in how to send back an echo reply. This document
defines a new reply mode, the Reply via Specified Path mode. This
new mode is used to direct the egress LSR of the tested LSP to send
the echo reply message back along the path specified in the echo
request message.
In addition, two new TLVs, the Reply Path TLV and Reply Traffic Class
(TC) [RFC5462] TLV, are defined in this document. The Reply Path TLV
contains one nested sub-TLV that can be used to carry the specified
return path information to be used by the echo reply message.
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these codes MUST be set to zero. Reply Path return code only used
when sending echo reply, and it MUST be ignored when processing echo
request message. This document defines the following Reply Path
return codes:
Value Meaning
------ ----------------------
0x0000 No return code
0x0001 Malformed Reply Path TLV was received
0x0002 One or more of the sub-TLVs in Reply Path TLV
was not understood
0x0003 The echo reply was sent successfully using the
specified Reply Path
0x0004 The specified Reply Path was not found, the echo
reply was sent via other LSP
0x0005 The specified Reply Path was not found, the echo
reply was sent via IP path
0x0006 The Reply mode in echo request was not set to 5(Reply
via Specified Path) although Reply Path TLV exists
0x0007 Reply Path TLV was missing in echo request
0x0008-0xfffb Not allocated, allocated via Standard Action
0xfffc-0xffff Experimental Use
Flag field is also 2 octets in length, it is used to notify the
egress how to process the Reply Paths field when performing return
path selection. The Flag field is a bit vector and has following
format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MUST be zero |A|B|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A (Alternative path): the egress LSR MUST select a non-default path
as the return path. This is very useful when reverse default path
problems are suspected which can be confirmed when the echo reply is
forced to follow a non-default return path. Here, the default path
refers to the path that the egress LSR will use to send the echo
reply when the return path is not explicitly specified as defined in
this document. If A bit is set, there is no need to carry any
specific reply path sub-TLVs, and when received, the sub-TLVs SHOULD
be ignored.
B (Bidirectional): the return path is required to follow the reverse
direction of the tested bidirectional LSP. If B bit is set, there is
no need to carry any specific reply path sub-TLVs, and when received,
the sub-TLVs SHOULD be ignored.
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The A bit and B bit set MUST NOT both be set, otherwise, an echo
reply with the RP return code set to "Malformed RP TLV was received"
SHOULD be returned.
The Reply Paths field is variable in length, not more than one sub-
TLV MUST be carried, which describes the specified path that the echo
reply message is required to follow. When the Reply Mode field is
set to "Reply via Specified Path" in an LSP echo request message, the
Reply Path TLV MUST be present.
3.3. Reply Path sub-TLVs
Each of the FEC sub-TLVs (include existing and future defined) for
the Target FEC Stack TLV[RFC4379] is applicable to be a sub-TLV for
inclusion in the Reply Path TLV for expressing a specific return
path. For these shared sub-TLVs, they share the same registry with
the Target FEC Stack TLV for the range of 0-31743 and 32768-64511.
In addition, this document defines three new sub-TLVs: IPv4 RSVP
Tunnel sub-TLV, IPv6 RSVP Tunnel sub-TLV and Static Tunnel sub-TLV.
These sub-TLVs are only designed for Reply Path TLV, hence this
document calls them dedicated sub-TLVs to Reply Path TLV. For these
dedicated sub-TLVs, this document will create a new registry (Section6.1), the sub-TLV type MUST be allocated from the new registry.
Detailed definition is in the following sections.
In [RFC4379], the range of 31744-32767 and 64512-65535 for sub-TLVs
is specified for Vendor Private Use, and MUST NOT be allocated. This
document changes that rule to make it not applicable to Reply Path
TLV and redefines the rule as in Section 6.2 . If an implementation
recognizes any specific Vendor Private types as defined in [RFC4379],
and uses the sub-TLV type specified in this document, care must be
taken to ensure that the implementation does not confuse the two
usages.
With the Return Path TLV flags and the sub-TLVs defined for the
Target FEC Stack TLV and in this document, it could provide following
options for return paths specifying:
1. Specify a particular LSP as return path
- use those sub-TLVs defined for the Target FEC Stack TLV
2. Specify a more generic tunnel FEC as return path
- use the IPv4/IPv6 RSVP and Static Tunnel sub-TLVs defined in
Section 3.3.1, Section 3.3.2 and Section 3.3.3 of this
document
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3. Specify the reverse path of the bidirectional LSP as return path
- use B bit defined in Section 3.2 of this document.
4. Force return path to non-default path
- use A bit defined in Section 3.2 of this document.
3.3.1. IPv4 RSVP Tunnel sub-TLV
The IPv4 RSVP Tunnel sub-TLV is used in the Reply Path TLV to allow
the operator to specify a more generic tunnel FEC other than a
particular LSP as the return path. According to the bits set in the
Flag field, the egress LSR will then choose an LSP from the specified
Tunnel as the return path. The format of IPv4 RSVP Tunnel sub-TLV is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 RSVP Tunnel sub-TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel end point address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel sender address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 IPv4 RSVP Tunnel sub-TLV
The IPv4 RSVP Tunnel sub-TLV is derived from the RSVP IPv4 FEC TLV
that is defined in Section 3.2.3 [RFC4379]. All fields have the same
semantics as defined in [RFC4379] except that the LSP-ID field is
omitted and a new Flag field is defined.
The IPv4 RSVP Tunnel sub-TLV Type field is 2 octets in length, and
the recommended type value is TBD.
The Flag field is 2 octets in length, it is used to notify the egress
LSR how to choose the return path. The Flag field is a bit vector
and has following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|P| MUST be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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P (Primary): the return path MUST be chosen from the LSPs that belong
to the specified Tunnel and the LSP MUST be the primary LSP.
S (Secondary): the return path MUST be chosen from the LSPs that
belong to the specified Tunnel and the LSP MUST be the secondary LSP.
P bit and S bit MUST NOT both be set, otherwise, an echo reply with
the RP return code set to "Malformed RP TLV was received" SHOULD be
returned. If P bit and S bit are both not set, the return path could
be any one of the LSPs from the same Tunnel.
3.3.2. IPv6 RSVP Tunnel sub-TLV
The IPv6 RSVP Tunnel sub-TLV is used in the Reply Path TLV to allow
the operator to specify a more generic tunnel FEC other than a
particular LSP as the return path. According to the bits set in the
Flag field, the egress LSR will then choose an LSP from the specified
Tunnel as the return path. The format of IPv6 RSVP Tunnel sub-TLV is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 RSVP Tunnel sub-TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel end point address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel sender address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 IPv6 RSVP Tunnel sub-TLV
The IPv6 RSVP Tunnel sub-TLV is derived from RSVP IPv6 FEC TLV that
is defined in Section 3.2.4 of [RFC4379].All fields have the same
semantics as defined in [RFC4379] except that the LSP-ID field is
omitted and a new Flag field is defined.
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The IPv6 RSVP Tunnel sub-TLV Type field is 2 octets in length, and
the type value is TBD.
The Flag field is 2 octets in length and is identical to that
described in Section 3.3.1.
3.3.3. Static Tunnel sub-TLV
The Static Tunnel sub-TLV is used in the Reply Path TLV to allow the
operator to specify a more generic tunnel FEC other than a particular
LSP as the return path. According to the bits set in the Flag field,
the egress LSR will then choose an LSP from the specified Tunnel as
the return path. The format of Static RSVP Tunnel sub-TLV is as
follows. The value fields are taken from the definitions in
[RFC6370].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Static Tunnel sub-TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Global ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Node ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Global ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Node ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Tunnel Num | Destination Tunnel Num |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flag | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Static Tunnel sub-TLV
The Flag field is 2 octets in length and is identical to that
described in Section 3.3.1.
The sub-TLV type value is TBD.
3.4. Reply TC TLV
Reply TOS Byte TLV [RFC4379] is used by the originator of the echo
request to request that an echo reply be sent with the IP header TOS
byte set to the value specified in the TLV. Similarly, in this
document, a new TLV: Reply TC TLV is defined and MAY be used by the
originator of the echo request to request that an echo reply be sent
with the TC bits of the return path LSP set to the value specified in
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this TLV. The format of Reply TC TLV is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply TC TLV type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TC | MUST be zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Reply TC TLV Type field is 2 octets in length, and the type value
is TBD.
The Length field is 2 octets in length, the value of length field is
fixed 4 octets.
4. Theory of Operation
The procedures defined in this document currently only apply to
"ping" mode. The "traceroute" mode is out of scope for this
document.
In [RFC4379], the echo reply is used to report the LSP checking
result to the LSP Ping initiator. This document defines a new reply
mode and a new TLV (Reply Path TLV) that enable the LSP ping
initiator to specify or constrain the return path of the echo reply.
Similarly the behavior of echo reply is extended to detect the
requested return path by looking at a specified path FEC TLV. This
enables LSP Ping to detect failures in both directions of a path with
a single operation, this of course cuts in half the operational steps
required to verify the end to end bidirectional connectivity and
integrity of an LSP.
When the echo reply message is intended to test the return MPLS LSP
path(when the A bit is not set in the previous received echo request
message), the destination IP address of the echo reply message MUST
never be used in a forwarding decision. To avoid this possibility
the destination IP address of the echo reply message that is
transmitted along the specified return path MUST be set to numbers
from the range 127/8 for IPv4 or 0:0:0:0:0:FFFF:127/104 for IPv6, and
the IP TTL MUST be set 1, and the TTL in the outermost label MUST be
set to 255. Of course when the echo reply message is not intended
for testing the specified return path (when the A bit is set in the
previous received echo request message) , the procedures defined in
[RFC4379] (the destination IP address is copied from the source IP
address) apply unchanged.
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Internet-Draft Return Path Specified LSP Ping August 20124.1. Sending an Echo Request
When sending an echo request, in addition to the rules and procedures
defined in Section 4.3 of [RFC4379], the reply mode of the echo
request MUST be set to "Reply via Specified Path", and a Reply Path
TLV MUST be carried in the echo request message correspondingly. The
Reply Path TLV includes one or several reply path sub-TLV(s) to
identify the return path(s) the egress LSR should use for its reply.
For a bidirectional LSP, since the ingress LSR and egress LSR of a
bidirectional LSP are aware of the relationship between the forward
and backward direction LSPs, only the B bit SHOULD be set in the
Reply Path TLV. If the operator wants the echo reply to be sent
along a different path other than the reverse direction of the
bidirectional LSP, the "A" bit SHOULD be set or another FEC sub-TLV
SHOULD be carried in the Reply Path TLV instead, and the B bit MUST
be clear.
In some cases, operators may want to treat two unidirectional LSPs
(one for each direction) as a pair. There may not be any binding
relationship between the two LSPs. Using the mechanism defined in
this document, operators can run LSP Ping one time from one end to
complete the failure detection on both unidirectional LSPs. To
accomplish this, the echo request message MUST carry (in the Reply
Path TLV) a FEC sub-TLV that belongs to the backward LSP.
4.2. Receiving an Echo Request
"Ping" mode processing as defined in Section 4.4 of [RFC4379] applies
in this document. In addition, when an echo request is received, if
the egress LSR does not know the reply mode defined in this document,
an echo reply with the return code set to "Malformed echo request"
and the Subcode set to zero will be send back to the ingress LSR
according to the rules of [RFC4379]. If the egress LSR knows the
reply mode, according to the Reply Path TLV, it SHOULD find and
select the desired return path. If there is a matched path, an echo
reply with Reply Path TLV that identify the return path SHOULD be
sent back to the ingress LSR, the Reply Path return code SHOULD be
set to "The echo reply was sent successfully using the specified
return path". If there is no such path, an echo reply with Reply
Path TLV SHOULD be sent back to the ingress LSR, the Reply Path
return code SHOULD be set to relevant code (defined Section 3.2) for
the real situation to reflect the result of Reply Path TLV processing
and return path selection. For example, if the specified LSP is not
found, the egress then chooses another LSP as the return path to send
the echo reply, the Reply Path return code SHOULD be set to "The
specified reply path was not found, the echo reply was sent via other
LSP", and if the egress chooses an IP path to send the echo reply,
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the Reply Path return code SHOULD be set to "The specified reply path
was not found, the echo reply was sent via IP path". If there is
unknown sub-TLV in the received Reply Path TLV, the Reply Path return
code SHOULD be set to "One or more of the sub-TLVs in Reply Path TLV
was not understood".
If the A bit of the Reply Path TLV in a received echo request message
is set, the egress LSR SHOULD send the echo reply along an non-
default return path.
IF the B bit of the Reply Path TLV in a received echo request message
is set, the egress LSR SHOULD send the echo reply along the reverse
direction of the bidirectional LSP.
If the A bit of the Reply Path TLV in a received echo request message
is not set(a.k.a a specific return path sub-TLV is carried or the B
bit is set), the echo reply is REQUIRED not only to send along the
specified path, but to test the selected return path as well (by
carrying the FEC stack information of the return path).
In addition, the FEC validate results of the forward path LSP SHOULD
NOT affect the egress LSR continue to test return path LSP.
4.3. Sending an Echo Reply
As described in [RFC4379], the echo reply message is a UDP packet,
and it MUST be sent only in response to an MPLS echo request. The
source IP address is a valid IP address of the replier, the source
UDP port is the well-know UDP port for LSP ping.
When the echo reply is intended to test the return path (the A is not
set in the previous received echo request), the destination IP
address of the echo reply message MUST never be used in a forwarding
decision. To avoid this problem, the IP destination address of the
echo reply message that is transmitted along the specified return
path MUST be set to numbers from the range 127/8 for IPv4 or 0:0:0:0:
0:FFFF:127/104 for IPv6, and the IP TTL MUST be set to 1, the TTL in
the outermost label MUST be set to 255. Otherwise, the same as
defined in [RFC4379], the destination IP address and UDP port are
copied from the source IP address and source UDP port of the echo
request.
When sending the echo reply, a Reply Path TLV that identifies the
return path MUST be carried, the Reply Path return code SHOULD be set
to relevant code that reflects results about how the egress processes
the Reply Path TLV in a previous received echo request message and
return path selection. By carrying the Reply Path TLV in an echo
reply, it gives the Ingress LSR enough information about the reverse
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direction of the tested path to verify the consistency of the data
plane against control plane. Thus a single LSP Ping could achieve
both directions of a path test. If the return path is pure IP path,
no sub-TLVs are carried in the Reply Path TLV.
4.4. Receiving an Echo Reply
The rules and process defined in Section 4.6 of [RFC4379] apply here.
When an echo reply is received, if the reply mode is "Reply via
Specified Path" and the Reply Path return code is "The echo reply was
sent successfully using the specified return path", and if the A bit
is not set. The ingress LSR MUST perform FEC validation (based on
the FEC stack information of the return path carried in the Reply
Path TLV) as an egress LSR does when receiving an echo request, the
FEC validation process (relevant to "ping" mode) defined in Section4.4.1 of [RFC4379] applies here.
When an echo reply is received with return code set to "Malformed
echo request received" and the Subcode set to zero. It is possible
that the egress LSR may not know the "Reply via Specified Path" reply
mode, the operator may choose to re-perform another LSP Ping by using
one of the four reply modes defined [RFC4379].
On receipt of an echo reply with Reply Path return code in the Reply
Path TLV set to "The specified reply path was not found, ...", it
means that the egress LSR could not find a matched return path as
specified. Operators may choose to specify another LSP as the return
path or use other methods to detect the path further.
5. Security Considerations
Security considerations discussed in [RFC4379] apply to this
document. In addition to that, in order to prevent using the
extension defined in this document for "proxying" any possible
attacks, the return path LSP MUST have destination to the same node
where the forward path is from.
6. IANA Considerations
IANA has a temporary allocation for a TLV from the "Multiprotocol
Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping
Parameters - TLVs" registry, "TLVs and sub-TLVs" sub-registry - Type
21 (Reply Path TLV). For this TLV the standards action sub-TLVs (the
range of 0-31743 and 32768-64511) shall be blocked from being
allocated. IANA is also requested to assign one new TLV from the
"Multiprotocol Label Switching Architecture (MPLS) Label Switched
Chen, et al. Expires February 28, 2013 [Page 16]